[Offline Transfer Exports] Display a list of available offline export history

What does this MR do and why?

Display a list of available offline export history at import/offline/export/history, not just the existing Empty State.

Only shows rows for which a bucket name and export prefix exists.

Follow up to make the list paginated -> #628920. I don't think that is required for beta release of the frontend right now.

References

#587078 (closed)

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Enable feature flags:

   offline_transfer_imports

   offline_transfer_exports

   offline_transfer_ui

2. In the app, toggle checkboxes for offline transfer in /admin/application_settings/general#js-import-export-settings

   1. Allow exporting GitLab groups and projects by offline transfer
   2. Allow importing GitLab groups and projects by offline transfer

3. Navigate to /import/offline/export/history to see the empty state

4. Create mock data in rails console with the following script

   user = User.find_by_username('root')   # your username
   org  = user.organizations.first || Organizations::Organization.first
   
   statuses = [
     { status: :finished, has_failures: false },  # Complete
     { status: :created,  has_failures: false },  # Pending
     { status: :started,  has_failures: false },  # Exporting…
     { status: :finished, has_failures: true  },  # Partially completed
     { status: :failed,   has_failures: false },  # Failed
   ]
   
   statuses.each_with_index do |attrs, i|
     export = Import::Offline::Export.new(
       user: user,
       organization: org,
       status: attrs[:status],
       has_failures: attrs[:has_failures],
       created_at: (i + 1).days.ago,
       updated_at: (i + 1).days.ago
     )
     export.save!(validate: false)
   
     Import::Offline::Configuration.create!(
       offline_export_id: export.id,
       organization_id: org.id,
       provider: :aws,
       bucket: "gitlab-exports-demo-#{i + 1}",
       source_hostname: 'https://gdk.test',
       object_storage_credentials: {}
     )
   end
   
   puts "Seeded: #{Import::Offline::Export.where(user: user).count} exports"
   

5. Reload the export history page.

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